A power module on which power semiconductor elements such as an insulated gate bipolar transistor (IGBT) are mounted is required to efficiently dissipate heat generated by the semiconductor elements to the outside while ensuring the insulation performance between the semiconductor elements and their surroundings. A conventional direct cooling-type power module not using grease is formed of: an insulated circuit board including a ceramic plate having main surfaces on which metal thin films are formed; a power semiconductor element joined onto one of the main surfaces with a joining material interposed therebetween; and a cooler joined onto the other main surface.
However, depending on the conditions that such a direct cooling-type power module is used, there is a large difference in coefficient of linear expansion between the semiconductor element and the cooler, thereby producing cracks in each of joining portions, with the result that the required heat dissipation performance may not be achieved. Thus, for example, in Japanese Patent National Publication No. 08-509844 (PTD 1), a semiconductor element is joined onto a conductor path on a ceramic substrate, and the ceramic substrate and a metal bottom plate below the ceramic substrate are joined to each other with a buffer layer interposed therebetween. The joining layer between the ceramic substrate and the buffer layer, and the joining layer between the buffer layer and the metal bottom plate each are made of sintered silver. Fatigues and crack formation caused by the difference in coefficient of linear expansion between the ceramic substrate and the metal bottom plate are avoided by plastic deformation of the buffer layer.
However, in PTD 1, there is no buffer layer provided between the semiconductor element and the ceramic substrate, which is thus less effective in diffusing the heat of the semiconductor element in the direction along its main surface. On the other hand, for example, Japanese Patent Laying-Open No. 2008-147469 (PTD 2) discloses a configuration in which a stress buffer plate is provided directly below a semiconductor element with a joining material interposed therebetween. This improves the effect of causing the heat generated by the semiconductor element to be diffused directly therebelow.